Halobutyl rubbers, which are halogenated isobutylene/isoprene copolymers, are the polymers of choice for best air retention in tires for passenger, truck, bus, and aircraft vehicles. Bromobutyl rubber, chlorobutyl rubber, and halogenated star-branched butyl rubbers can be formulated for specific tire applications, such as tubes or innerliners. The selection of ingredients and additives for the final commercial formulation depends upon the balance of the properties desired, namely, processability and tack of the green (uncured) compound in the tire plant versus the in-service performance of the cured tire composite. Examples of halobutyl rubbers are bromobutyl (brominated isobutylene-isoprene rubber or BIIR), chlorobutyl (chlorinated isobutylene-isoprene rubber or CIIR), star-branched butyl (SBB), EXXPRO™ elastomers (brominated isobutylene-co-p-methyl-styrene) copolymer (otherwise known as BIMSM), etc.
For rubber compounding applications, traditional small sub-micron fillers such as carbon black and silica are added to halobutyl rubbers to improve fatigue resistance, fracture toughness and tensile strength. More recently, methods to alter product properties and improve air barrier properties in halobutyl rubbers have been developed that comprise adding nanofillers apart from these traditional fillers to the elastomer to form a “nanocomposite.” Nanocomposites are polymer systems containing inorganic particles with at least one dimension in the nanometer range (see, for example, WO 2008/042025).
Common types of inorganic particles used in nanocomposites are phyllosilicates, an inorganic substance from the general class of so called “nanoclays” or “clays” generally provided in an intercalated form wherein platelets or leaves of the clay are arranged in a stack in the individual clay articles with interleaf spacing usually maintained by the insertion of another compound or chemical species between the adjacent lamellae. Ideally, intercalation inserts into the space or gallery between the clay surfaces. Ultimately, it is desirable to have exfoliation, wherein the polymer is fully dispersed with the individual nanometer-size clay platelets.
The extents of dispersion, exfoliation, and orientation of platy nanofillers such as organosilicates, mica, hydrotalcite, graphitic carbon, etc., strongly influence the permeability of the resulting polymer nanocomposites. The barrier property of a polymer in theory is significantly improved, by an order of magnitude, with the dispersion of just a few volume percent of exfoliated high aspect-ratio platy fillers, due simply to the increased diffusion path lengths resulting from long detours around the platelets. Nielsen, J. Macromol. Sci. (Chem.), vol. A1, p. 929 (1967), discloses a simple model to determine the reduction in permeability in a polymer by accounting for the increase in tortuosity from impenetrable, planarly oriented platy fillers. Gusev et al., Adv. Mater., vol. 13, p. 1641 (2001), discloses a simple stretched exponential function relating the reduction of permeability to aspect ratio times volume fraction of the platy filler that correlates well with permeability values numerically simulated by direct three-dimensional finite element permeability calculations.
To maximize the effect of aspect ratio on permeability reduction, it is therefore useful to maximize the degree of exfoliation and dispersion of the platelets, which are generally supplied in the form of an intercalated stack of the platelets. However, in isobutylene polymers, dispersion and exfoliation of platy nanofillers requires sufficient favorable enthalpic contributions to overcome entropic penalties. As a practical matter, it has thus proven to be very difficult to disperse ionic nanofillers such as clay into generally inert, nonpolar, hydrocarbon elastomers. The prior art has, with limited success, attempted to improve dispersion by modification of the clay particles, by modification of the rubbery polymers, by the use of dispersion aids, and by the use of various blending processes.
Due to the difficulties encountered in dispersing ionic nanoclays in nonpolar elastomers, graphitic carbon has been explored as an alternative platy nanofiller. For example, elastomeric compositions comprising graphite nanoparticles are described in U.S. Pat. No. 7,923,491.
US 2006/0229404 discloses a method for making compositions of an elastomer with exfoliated graphite in which the diene monomers are polymerized in the presence of 10 phr or more exfoliated graphite so that the graphite is intercalated with the elastomer.
U.S. Pat. No. 8,110,026 describes a process for producing a functional graphene sheet (FGS) based on exfoliation of oxidized graphite suitable for a high degree of dispersion in a polymer matrix for use in a nanocomposite.
Nano graphene platelets (NGPs) obtained through rapid expansion of graphite have become commercially available as of late. These NGPs have graphitic surfaces, as opposed to graphene oxide platelets of oxidized graphitic surfaces, and are quite compatible with hydrocarbon based non-polar butyl halobutyl rubbers. However, a high degree of exfoliation and dispersion of NGPs without agglomerations and aggregations cannot be achieved by solid compounding or solution mixing of these nanoparticles into halobutyl rubbers.
There is a need, therefore, for improving the dispersion of graphite and graphene nanofillers in elastomeric nanocomposite compositions comprising halobutyl rubbers useful for tires, air barriers, and other things requiring air retention, in order to improve the air impermeability of those compositions. The present invention fulfills this need by providing a novel graphite and graphene nanofiller dispersant useful in isobutylene-based elastomer/nanofiller nanocomposite compositions that results in these nanocomposite compositions having improved air barrier properties and that are suitable for use as a tire innerliner or innertube.